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Evolution, Extinction and the Fossil Record Life of the Earth

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Evolution, Extinction and the Fossil Record Life of the Earth 7 Articles atmosphere, taking into account temperature, cloud cover, wind and rainfall. They found the warmer eddies in the Southern Ocean were typically 0.5°C warmer than their surroundings and the cold ones were 0.5°C colder than the water beyond. Throughout the South- ern Ocean, these temperature anomalies correspond to changes in cloud cover and water content, as well as the frequency and prob- ability of rainfall. The reason? They alter the flux of heat between the ocean and the atmosphere. In the atmosphere, low pressure systems are the ones that generate rainfall. As these systems pass cold-core eddies, which have less heat to release, the cloud cover drops, moisture declines and rainfall reduces by 2-6%. The converse is true for warm-core eddies, which stimulate rainfall in their local vicinity. As well as providing the fuel for rain-filled clouds, the oceans shape where rain forms and falls around the planet. The heat energy heist undertaken by the Northern Hemisphere as it harvests energy from the south drives the differences in rainfall either side of the equator. Eddies bud off large ocean currents, like the Gulf Stream, bringing packages And an eddy in the ocean is all that’s needed to create a small, but of cold water into warmer regions and pockets of warmer water into colder significant, change in the amount of rainfall close to the water mass. regions. These 10–100 km across masses of water have a big impact on local Combined, these studies highlight just how wonderfully connected wind and rainfall patterns. The colour is indicative of relative water tempera- ture. (Credit: National Science Foundation/NOAA) the waters of the Earth are. Sara Mynott An eddy can be a package of warm water in an otherwise cold EGU Communications Officer ocean, or a pocket of cold in the warm, and they affect the atmos- phere differently, depending on their temperature. References Fasullo, J.: Rainfall’s oceanic underpinnings, Nature Geosci., 6, 901–902, The effect of an eddy on atmospheric circulation has, until now, 2013 been rather overlooked, as they were initially thought to be too small Frenger, I. et al.: Imprint of Southern Ocean eddies on winds, clouds and to have a significant impact. A group of Swiss scientists set out to rainfall, Nature Geosci., 6, 608–612, 2013 solve the mystery of how eddies effect the atmosphere by looking Frierson, D. M. et al.: Contribution of ocean overturning circulation to tropical at some 600,000 eddies in the Southern Ocean. Using satellite rainfall peak in the Northern Hemisphere, Nature Geosci., 6, 940–944, 2013 data, they tracked both eddies and the properties of the surrounding Evolution, extinction and the fossil record Life of the Earth Recent research suggests that the Earth is entering a biodiversity and we may be able to reduce extinction rates by targeting conser- crisis and we may be on the brink of our planet’s sixth mass extinc- vation where it is most needed, or will be most effective. tion. Under these circumstances, an understanding of macroevo- lutionary patterns in diversification and extinction will be vital to Uncertainty remains in these estimates, though. To get a clear pic- guide conservation strategies. In a recent analysis, Barnosky and ture of which conservation approaches could avoid a mass extinc- colleagues from the University of California showed current extinc- tion, it is important to address this uncertainty. A huge amount of tion rates are highly elevated when compared to background rates information is available on present day extinction patterns and risk in the geological past. If extinctions continue at this pace, we could factors, but this is only a snapshot of the 3.5 billion years that life be seeing an event that qualifies as the Earth’s sixth mass extinc- has existed on Earth. To map out and truly understand macroevo- tion (defined by a loss of at least 75% of species) in as little as lutionary processes that could help us today, we have to use data 300 years from now. Fortunately, the team also discovered that it from fossils, and the researchers emphasise that integration of pal- may not be too late to slow down extinction rates and avoid a cata- aeontological and present day data will be crucial. Previously, the strophic event. So far we have only lost a few percent of species, differences between these types of data have often made it difficult GeoQ Issue 9 8 Articles to use both of them in combination. However, advances in methods used to analyse the fossil record, and an increase in data available from ongoing collection efforts, have led to interesting insights into the interrelated factors that have shaped the diversity of life on Earth as we see it today. In their paper, Barnosky’s team outline the key complications in merging fossil and recent data, the most apparent of which is time- scale. Modern data goes back a few hundred years at the most, whereas fossil data are recorded on geological timescales, and the majority of past mass extinction events are estimated to have occurred over millions of years. The apparent rate of extinction var- ies with the time period over which it is measured, so this is a par- ticularly important problem to solve. Other difficulties include the sparsity of fossil data: estimated sampling rates are around 70% for the very best preserved fossil Extinction magnitudes in the geological past and the present day. Numbers groups, but are generally more like 10% at the most. Out of the spe- next to each icon indicate the percentage of species extinct. White icons indi- cies that we have discovered, many are in fact only known from cate species ‘extinct’ and ‘extinct in the wild’ over the past 500 years. Black icons add currently ‘threatened’ species to those already ‘extinct’ or ‘extinct in a single specimen (and sometimes a single tooth!). Some ancient the wild’. Yellow icons indicate the big five mass extinctions, with a black line at groups had high preservation potential – such as marine gastro- the 75% benchmark for comparison. (Reprinted by permission from Macmillan pods and bivalves – but, unfortunately, these groups have received Publishers Ltd: Nature, Barnosky et al., 2011) far less research attention in the present day than larger and more easily accessible groups like birds and mammals. These new methods can be used to untangle the relative impor- Fortunately there is some hope for improving the basis for com- tance of factors such as body size, population size and ecological parison, in the form of phylogenies. A phylogeny is a hypothesis of specialism, so that we can begin to identify the species most at risk the evolutionary relationships between species, represented as an of extinction, and those whose survival will be most effective for evolutionary ‘tree’ with branches scaled either to the time the spe- maintaining biodiversity. To move forward from these advances we cies lived for, or the amount of evolutionary change along a branch. must incorporate them into the effort to standardise different types In a 2008 review focussing on understanding patterns in extinc- of data, and into developing predictive models of how different risk tion, Andy Purvis summarised the progress that has been made in factors interact. uncovering rates of speciation and extinction using phylogeny as a framework. He also showed how extinction patterns relate to phy- A final and particularly interesting idea to come out of Barnosky’s logenetic relationships between species (i.e. do close relatives go research is that of the ‘perfect storm’. Past mass extinctions often extinct at the same time?), in different scenarios. Phylogeny can be seem to have occurred during synergies of unusual events. The used to test whether factors like the geographic range of a species Earth’s current changing climate dynamics, in combination with new or a particular physical trait correlate with extinction proneness. This ecological stressors like habitat fragmentation, pollution, overfish- is often done in present day analyses, but less so for the geological ing and invasive species, may represent such a synergy. The fossil past. He finished by calling for more work in improving methods to record could act as an ideal natural laboratory to formulate, model model combined data sets in a phylogenetic framework. and test this hypothesis. Although the Earth has recovered from cat- astrophic extinction events in the past, it has never before supported In a review published last year, Graham Slater and Luke Harmon 7 billion humans, and modelling macroevolutionary patterns in order summarise innovative new methods based upon phylogeny that are to help mitigate against escalating extinction threats may be key in being employed to help tackle some of these data comparison and determining our own future as a species. modelling problems. One such method can be used to stochasti- cally scale evolutionary trees of fossil taxa to time, and will allow Laura Soul palaeobiologists to more accurately estimate the timescales of evo- Postgraduate Student at the Department of lutionary change and extinction in the fossil record. This, in turn, will Earth Sciences, University of Oxford enable them to make valid comparisons with the rate of events in the present day. In addition, Slater and Harmon point to several new References methods that will allow researchers to develop and analyse phylog- Barnosky, A. D. et al.: Has the Earth’s sixth mass extinction already arrived?, enies that include similar numbers of extinct and living species, and Nature, 471, 51–57, 2011 to more accurately model phenotypic change using fossil and living Purvis, A.: Phylogenetic approaches to the study of extinction, Annu. Rev.
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